1. Field of the Invention
This invention relates to a process for forming a duplex coating on the surface of an aluminum substrate such as sheet. More particularly, this invention relates to a protective coating on aluminum which comprises an aluminum hydroxide layer on the metal surface and a functionalized layer of phosphonic/phosphinic acid chemically bonded to the hydroxide layer.
2. Description of the Related Art
It is well known that the natural oxide on aluminum can be reinforced or altered by several treatment methods to give better protection against corrosion or serve as a substrate for adhesive bonding. Such treatments include anodic oxidation, hydrothermal treatment in water, water vapor or aqueous solutions, or reaction treatments such as conversion coatings which employ solutions which may contain chromic acid, chromates, phosphoric acid, phosphates and fluorides.
While anodic oxides can be grown to thicknesses of tens of microns, their application is generally time consuming and requires expensive electrical equipment. Hydrothermal coatings are easier and faster to apply and require only simple equipment. They consist mostly of AlOOH or Al(OH).sub.3 in crystalline or amorphous (gelatinous) form and are generally known as boehmite coatings as disclosed in Wernick et al, "The Surface Treatment and Finishing of Aluminum and Its Alloys", ATM Publ., 1987. These coatings offer reasonably good protection in mildly aggressive environments. Chemical conversion coatings, e.g., Alodine coatings as disclosed in Wernick et al, can be applied in several seconds to a few minutes. They afford good corrosion protection and are good bonding substrates for polymer coatings, laminates or adhesives although they do not match the adhesion and durability performance of anodic oxides applied in phosphoric acid according to U.S. Pat. No. 4,085,012. Their disadvantage is their content of Cr-VI (chromate) compounds which require costly disposal of residues (sludge).
Venables et al, U.S. Pat. No. 4,308,079, teach the treatment of an aluminum oxide surface of an aluminum substrate with a partial monomolecular layer of an amino phosphonate compound such as nitrilotris (methylene) triphosphonic acid to retard hydration of the aluminum oxide to aluminum hydroxide to provide a more stable microporous surface which is particularly suited to commercial adhesives. The formation of the hydrated oxide is said to interfere with the stability of a satisfactory bond between the adhesive and the oxide, while the phosphonate treatment to form a partial monomolecular layer is said to be sufficient to inhibit the conversion of the oxide to hydroxide without interfering with subsequent bonding of the adhesive to the oxide.
In the present invention, it has been discovered that a duplex coating can be applied to a metal surface. The coating comprises a layer of metal hydroxide, e.g., aluminum hydroxide, and a layer which is comprised of the reaction product of phosphonic or phosphinic acids such as monomeric or polymeric acids.